Infiltrated aluminum preforms

ABSTRACT

A method for the manufacture of a three-dimensional object includes the steps of forming a mixture that contains a binder and a least one of aluminum or a first aluminum-base alloy into a green composite, removing the binder from said green composite, forming a porous preform structure, reacting the aluminum or first aluminum base alloy with nitrogen to form a rigid skeleton and infiltrating the porous structure with molten aluminum or second aluminum base alloy to form the three-dimensional object with near theoretical density. The green composite may be formed by an additive process such as computer aided rapid prototyping, for example selective laser sintering. The method facilitates the rapid manufacture of aluminum components by an inexpensive technique that provides high dimensional stability and high density.

CROSS REFERENCE TO RELATED APPLICATION(S)

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 10/801,761 for “Infiltrated Aluminum Preforms” thatwas filed on Mar. 15, 2004 and is a continuation-in-part of U.S. patentapplication Ser. No. 10/260,158 for “Infiltrated Aluminum Preforms” thatwas filed on Sep. 27, 2002, now U.S. Pat. No. 6,823,928. Both U.S. Ser.No. 10/801,761 and U.S. Pat. No. 6,823,928 are incorporated by referencein their entireties herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method for the rapid manufactureof aluminum alloy components and can have specific application tolimited production runs, such as those encountered in rapid prototypingor rapid manufacturing. More particularly it relates to a method wherebya porous aluminum or first aluminum alloy preform is formed into adesired shape and supported by a polymeric binder. A second aluminumalloy having a melting point equal to or greater than the aluminum orfirst aluminum alloy is caused to infiltrate the porous preform forminga dense structurally sound component.

2. Description of the Related Art

Aluminum and aluminum alloy components are traditionally fabricated bycasting, mechanical working or machining, as well as combinations ofthese processes. When casting, molten metal fills a mould having aninternal cavity formed into the shape of a desired component. After themolten metal cools and solidifies, the component is removed from themould in either net shape (finished form) or near net shape (close tofinished form, but requiring some additional working or machining). Whenmechanical working, such as forging, drawing, rolling, extrusion orstamping, a cast billet of the metal is mechanically deformed into theshape of the desired component. Casting requires moulds machined to theshape of the desired component while tools used to apply mechanicaldeformation require dies formed to the required shape. While bothcasting and mechanical working are well suited for the economicalmanufacture of large quantities of identically shaped components,neither is particularly suitable for specialty applications orprototypes where only a few components are required or where variousaspects of the shape are to be varied from component to component.

Aluminum and aluminum alloy components can also be machined from stockmaterial that may have been mechanically worked or cast. Machining istime consuming, has a very poor utilization of raw material and requiresskilled operators. Machined components are therefore expensive and slowto produce.

Aluminum powders can also be used to fabricate parts, either to takeadvantage of a unique property set or because net shape parts can beinexpensively fabricated. In the former case, the powders are typicallyconsolidated by extrusion, forging or hot isostatic pressing. Net shapeparts are made by pressing aluminum powder at extremely high pressures(nominally in excess of 30,000 pounds per square inch) into hard toolingcavities to achieve green densities as high as 95%. After pressing, thepart is ejected from the die and the so-called green body is sintered ina furnace at elevated temperatures under a controlled atmosphere,commonly nitrogen. Aluminum, and aluminum alloys, have a propensity toform a highly stable alumina (Al₂O₃) surface film that passivates theindividual powder particles limiting further oxidation. The surfaceoxide also hinders the diffusional mechanisms needed to sinter aluminumpowder preforms into fully dense aluminum components. As a solution tothis challenge, the aluminum powder industry has developed blends ofaluminum powder, surface oxide reducing agents, lubricants and sinteringagents. All of these technologies require tooling or dies which are usedto shape the part. This tooling is expensive and is time consuming toproduce. This delays the time needed to introduce new products andincreases their cost.

An alternative production strategy produces three-dimensional objectsdirectly from the manipulation of data from computer aided design (CAD)databases. Various technologies are known to produce such parts,particularly through the use of additive processes as opposed tosubtractive processes such as conventional machining. Important additiveprocesses for the production of such parts include stereolithography,selective laser sintering, laminated object manufacturing,three-dimensional printing and fused deposition modeling. A commonfeature of all of these rapid prototyping and rapid manufacturingtechniques is that energy and/or material is delivered to a point toproduce a solid. A series of lines are then traced out to make across-sectional layer and a series of layers formed to make a threedimensional part. In principle, there are as many such potentialmanufacturing systems as there are ways to write or draw on a surface.Producing components in this way has a number of important advantagesover traditional manufacturing processes. Most importantly, parts of anyshape can be produced directly from a CAD model without the need forexpensive tooling or machining and these can be produced in a smallfraction of the time that is typically required of traditionalmanufacturing operations.

Selective laser sintering is described in more detail in U.S. Pat. No.4,863,538 to Deckard and three-dimensional printing is described in moredetail in U.S. Pat. No. 6,416,850 to Bredt, et al. Both the U.S. Pat.No. 4,863,538 patent and the U.S. Pat. No. 6,416,850 patent areincorporated by reference in their entireties herein. These techniqueshave been used to fabricate objects made from a variety of materialssuch as photoset resins, other polymers such as nylon and ethylenebutadiene styrene, organic waxes, ceramics such as SiN, and metals, mostcommonly steel.

Recently, aluminum parts have been produced by selective laser sinteringand extrusion freeform fabrication. These aluminum parts were fabricatedas polymer/aluminum powder composites and post-processed by burning outthe polymer and then sintering the remnant metal powder to full ornear-full density, in a manner similar to that used in powder injectionmolding. However, it is extremely difficult to maintain dimensionalaccuracy during sintering of such a powder preform because of densitygradients in the green part and geometrical constraints. While uniformshrinkage can be incorporated into the initial CAD design, non-uniformshrinkage, or distortion, is more difficult to control reproducibly andto accommodate by design. Because dimensional accuracy is a criticalcriterion for any rapid prototyping/rapid manufacturing system, theinability to accurately sinter large parts is fatal. Only small aluminumparts can presently be made this way: the limit is approximately 1 cm³.

U.S. Pat. No. 4,828,008 discloses that a permeable ceramic mass isspontaneously infiltrated by a molten aluminum alloy containing at least1%, by weight, of magnesium and optionally also containing silicon.“Spontaneous infiltration” means that the molten metal infiltrates thepermeable mass without the requirement for the application of pressureor vacuum (whether externally applied or internally created). U.S. Pat.No. 4,828,008 is incorporated by reference in its entirety herein.

U.S. Pat. No. 5,020,584 discloses that spontaneous infiltration of apreform formed from a mixture of a powdered matrix metal and a powderfiller (ceramic) with a molten metal. It is disclosed that when thepowdered matrix metal is aluminum and the filler aluminum oxide, theinfiltrating atmosphere forms a skin (such as an oxide or nitrogencompound) on the metal that prevents particle separation. U.S. Pat. No.5,020,584 is incorporated by reference in its entirety herein.

The dimensional accuracy of a component formed is much improved byinfiltration, whether spontaneous, pressure-assisted or vacuum-assisted.The loosely formed powder body is lightly pre-sintered and the porousmass is subsequently infiltrated by a liquid metal at a temperaturebetween the melting point of the infiltrant and the base metal. Becausethere is so little sintering, there is negligible dimensional changebetween the preform and the finished part. Numerous systems have beenfabricated by the rapid prototyping/rapid manufacturing/infiltrationroute to date, including Fe—Cu, stainless steel-bronze, ZrB₂—Cu andSiC—Mg. Aluminum and aluminum base alloys are a conspicuous omissionfrom the successful metallic infiltration systems. It is theorized thatthe alumina surface film on the aluminum and aluminum alloy particlesmay have prevented the infiltration of porous aluminum components.

There remains, firstly, a need for a method to spontaneously infiltratea porous mass of a first aluminum-base material with a molten secondaluminum-base material. In addition, there remains a need for anadditive process to manufacture aluminum alloy parts that does not havethe above-stated deficiencies. The additive process should result inparts with a density close to the theoretical density of the aluminumalloy and be capable of a high level of dimensional accuracy.

BRIEF SUMMARY OF THE INVENTION

In accordance with a first embodiment of the invention, there isprovided a method for the spontaneous infiltration of a porousaluminum-base preform. This method includes the steps of forming amixture that contains a binder and at least one of aluminum or a firstaluminum-base alloy into a green composite, removing the binder from thegreen composite forming a porous preform structure, converting a portionof the aluminum to a rigid aluminum nitride skeleton and infiltratingthe porous preform structure with a molten second aluminum base alloy toform the three-dimensional object with near theoretical density.

In accordance with a second embodiment of the invention, there isprovided a method for the manufacture of a three-dimensional object thatincludes the steps of forming a mixture that contains a binder and atleast one of aluminum or a first aluminum-base alloy into a greencomposite, removing the binder from the green composite forming a porouspreform structure, converting a portion of the aluminum to a rigidaluminum nitride skeleton and infiltrating the porous preform structurewith a molten second aluminum base alloy to form the three-dimensionalobject with near theoretical density.

The green composite may be formed by an additive process such ascomputer aided rapid prototyping, for example selective laser sinteringor a casting or molding process such as a room temperature vulcanizationprocess like the Keltool® process, metal injection molding, extrusionmolding, resin transfer molding, rotational molding, or pressing. Themethod facilitates the manufacture of small numbers of aluminumcomponents by an inexpensive technique that provides high dimensionalstability and high density.

For either the first embodiment or the second embodiment, an aluminumnitride skeleton is formed on the surfaces of the aluminum particles orthe particles of a first aluminum-base alloy powder for increasedpreform strength and dimensional stability. It is a feature of theinvention that an enhanced surface finish may be achieved by utilizing anitrogen based atmosphere for the converting step and a noble gas basedatmosphere, such as argon, for the infiltrating step. It is a furtherfeature of the invention that infiltration can be achieved byinfiltrating at a temperature in excess of the melting point of thealuminum or first aluminum based alloy used to make the green composite.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in flow chart representation a process for the infiltrationof a porous aluminum-base preform by a molten aluminum-base alloy.

FIG. 2 graphically illustrates the effect of nitride skin thickness ontensile ductility of a composite.

FIG. 3 graphically illustrates the rate of nitride formation as afunction of both temperature and the presence of tin.

FIG. 4 graphically illustrates the rate of nitride formation as afunction of tin content.

FIG. 5 graphically illustrates the effect of tin on the infiltrationrate.

FIG. 6 shows in cross-sectional representation an aluminum alloyparticle used to form the porous mass in accordance with an embodimentof the invention.

FIG. 7 shows in cross-sectional representation a blend of powders inaccordance with an embodiment of the invention.

FIG. 8 shows in cross-sectional representation a first assembly forinitial heating of the blend of powders shown in FIG. 7.

FIG. 9 shows in cross-sectional representation the effect of nitrationon the blend of powders shown in FIG. 8.

FIG. 10 shows in cross-sectional representation a second assembly forinitial heating of the blend of powders shown in FIG. 7.

FIG. 11 illustrates a bleed-out problem solved by an embodiment of theinvention.

Like reference numbers and designations in the various drawingsindicated like elements.

DETAILED DESCRIPTION

FIG. 1 shows in flow chart representation a sequence of process steps 10in accordance with a first embodiment of the invention. In a first step12, a mixture containing a binder and at least one of aluminum or afirst aluminum-base alloy is formed into a green composite. As usedherein, “aluminum-base” means that the alloy contains at least 50%, byweight, of aluminum.

Preferably the mixture comprises a minimum of three powders blendedtogether. One powder is an aluminum alloy, such as aluminum alloy 6061,that constitutes approximately about 80 to about 95%, by volume, andmore preferably from about 85% to about 90%, by volume, of the totalmixture. This base metal may be any aluminum-base alloy powder or purealuminum.

The aluminum or first aluminum-base alloy is in powder form of anyeffective particle size. Preferably the average particle size is betweenabout 5 and about 150 microns (1 micron=1 μm=1×10⁻⁶ meter) and morepreferably, the average particle size of the metallic powder is in therange of from about 10 to about 75 microns. One preferred aluminum alloyis aluminum alloy 6061 that has a nominal composition, by weight, ofabout 0.4% to about 0.8% silicon, about 0.15% to about 0.40% copper,about 0.8% to about 1.2% magnesium, about 0.04% to about 0.35% chromiumand the balance aluminum and unavoidable impurities.

As a non-limiting list, the aluminum may be alloyed with one or more ofthe following elements tin, lead, bismuth, indium, antimony, copper,magnesium, silicon, zinc, titanium, chromium, zirconium, nickel, iron,manganese and silver.

Tin is preferred, as an alloying addition of tin improves the wetting ofthe infiltrating liquid and controls the nitride growth rate. The amountof nitride that forms is a critical parameter that controls bothinfiltration and properties of the finished product. If insufficientnitride forms, then dimensional stability is compromised duringinfiltration. Excess nitride detrimentally impacts the mechanicalproperties, such as tensile ductility. With reference to FIG. 2, theweight gain of an aluminum-2% magnesium-1% silicon preform that isinfiltrated with aluminum-5% magnesium is assumed due to the formationof nitride. As the weight increases, the tensile ductility decreases.Excess nitride can also cause cracking that can limit the production oflarge parts.

Because nitride growth is important, rapid nitride growth rates make theprocess difficult to control by decreasing the size of the operationalwindow and increasing costs. An addition of tin facilitates control ofthe nitride growth. With reference to FIG. 3, the amount of nitride thatforms and the rate at which the nitride grows are dependent on thereaction time and temperature. Without tin at either 540° C. or 570° C.,the initial rate is reasonably slow; however the rate of nitrideformation accelerates after approximately four hours. With tin, the rateof growth does not change over time and therefore the amount of nitridethat forms in a given time is less when tin is added to the powdermixture than when it is not.

The effect of tin concentration on the weight gain due to nitridation isshown in FIG. 4. A tin content as low as 0.1%, by weight, is effectiveand a content of between 0.5% and 2.0%, by weight, is preferred.

With reference to FIG. 5, the tin also affects the infiltration rate.Without tin, there is an incubation period. This incubation period isabsent in the presence of tin. The maximum infiltration rate is alsogreater in the presence of tin than without any tin present.

Tin powder may be mixed with the aluminum powder or prealloyed into thealuminum powder. Either embodiment is effective to control nitridegrowth rate. Prealloying tends to reduce variability in tensileproperties, presumably because there is less segregation than whenpowders are mixed.

Other alloying additions also affect the growth of the nitride skeleton.As a non-limiting exemplary list, these other elements include lead,bismuth, indium, antimony, selenium and silicon. In the absence of tin,the nitride formation rate is highest for pure aluminum and lower foraluminum containing additions of silicon and magnesium, referred to asaluminum alloys of the 6xxx series, where x is between 0 and 9.

In addition to aluminum alloy 6061, the following aluminum alloys havebeen shown to reduce the rate of nitride formation and are examples ofthe preferred first aluminum-base alloy: 6063, nominal composition byweight, Al-0.7% Mg-0.4% Si; 6082, nominal composition by weight Al-0.9%Mg-1.0% Si-0.7% Mn; 6106, nominal composition by weight Al-0.6% Mg-0.45%Si-0.25% Cu and 6351 nominal composition by weight Al-0.6% Mg-1.0%Si-0.6% Mn. Other aluminum alloys may be used in the presence of tin.

A second powder component of the mixture, constitutes approximatelyabout 0.1 to about 4%, by weight, and preferably from about 1.5% toabout 2.5%, by weight of the total mixture. This second powder is anoxygen-scavenger, such as magnesium that cleans the atmospheresurrounding the mixture and reduces the oxide layer on the aluminum-basemetal particles. The oxygen scavenger is in particle form with anaverage particle size of between about 5 and about 150 microns andpreferably the average particle size is between about 10 and about 75microns. Other suitable oxygen scavengers include zirconium, lithium,beryllium, calcium, cerium, lanthanum, neodymium, praseodymium,samarium, thorium, uranium, or misch metal or aluminum alloyed withmagnesium, zirconium, lithium, beryllium, calcium, cerium, lanthanum,neodymium, praseodymium, samarium, thorium, uranium, or misch metal.

As disclosed herein below, while there are benefits to including theoxygen scavenger in the powder mixture, effective infiltration is alsoobtained if the oxygen scavenger is included in a support layer formedabout the green composite, in which case the oxygen scavenger may beomitted from the powder mixture.

A third powder component of the mixture, which constitutes approximately5-15%, by volume, and preferably about 8% to about 12%, by volume, ofthe total blend, includes a binder. The binder may be an organo-metallicbinder such as zinc stearate, an organic or an inorganic binder, butpreferably is an organic polymeric binder. Organic polymeric binders caninclude thermoplastics with sharp melting points such as nylon 6, nylon11, nylon 12, copolymers of nylon 12 and nylon 6, polyacetals,polyethylene, polyethylene copolymers, polymethacrylates, polypropylene,and polyether block amides. The average particle size of the polymerbinder powder is generally in the range of about 1 to about 50 μm. Thebinder is selected to be a material that decomposes to a gas with aminimum of carbon residue when heated to a temperature of between about300° C. and about 500° C. in a suitable atmosphere, such as nitrogen.Where nitrogen is used to create the atmosphere, the nitrogen source canbe pure nitrogen, a mixture of gases including nitrogen, any nitrogenrich binder material that evolves nitrogen to form a nitride skeleton,metal nitrides such as transition metal nitrides or magnesium nitride.

The green composite is heated relatively slowly, such as about 1 toabout 2° C. per minute to facilitate binder vapor outgassing from thegreen composite by way of open, connected, porosity without a build upof localized pockets of vapor pressure that could damage the part.Preferred binders have a relatively low melt flow viscosity (on theorder of 25 to 145 grams per 10 minutes) and substantially completedecomposition to nitrogen, nitrogen compounds and other gases whenheated to a temperature in the 300°-500° C. range. Suitable bindersinclude the aforementioned nylons, and more specifically Orgasol® 2001Nylon-12 (gram molecular weight of 17,400, melt flow viscosity of about25 to about 100 grams per 10 minutes and decomposition temperature ofabout 433 to about 481° C.), Orgasol® 3501 EXD (gram molecular weight of6,500, melt flow viscosity of about 25 to about 100 grams per 10 minutesand decomposition temperature of about 414 to about 472° C.) andOrgasol® 3501UD (melt flow viscosity of about 25 to about 100 grams per10 minutes and decomposition temperature of about 425 to about 472° C.).All of these aforementioned Orgasol® nylon binders are availablecommercially from Atofina SA, of 4-8, cours Michelet-La Défense10-F-92800 Puteaux-France.

A homogeneous green composite of the powders having a desired shape isformed either by deposition in a mould or by using any rapid prototypingtechnique, such as those described above. A resin bonded preform isformed by exposing the green composite to a suitable cure initiator,such as heat or ultra-violet light. This resin bonded preform has adensity of approximately about 40% to about 70% of the theoreticaldensity for the aluminum alloy and structurally has an interconnectingnetwork of pores extending through the preform.

The binder is next removed, as illustrated in FIG. 1 by numeral 14, suchas by thermal decomposition, by heating to a temperature in the range offrom about 300° C. to about 500° C. with a sufficiently low heat up rateto avoid the formation of high pressure vapor pockets. Removal of thepolymer binder will temporarily reduce the integral strength of thepart. To provide support, a support powder that will not bond to thepart under the processing conditions surrounds the preform. Suitablesupport powders include ceramics, such as alumina, silicon carbide andboron nitride, mixed with an oxygen scavenger, such as magnesium. Asnoted above, inclusion of an oxygen scavenger in the support powder maybe sufficiently effective to remove the need to include an oxygenscavenger in the mixture of powders forming the green composite.

The oxygen scavenger is present in an amount of from about 0.1% to about10%, by volume, of the support powder, and more preferably, is presentin an amount of from about 0.5% to about 5%, by volume. A preferredscavenger is a magnesium powder. The magnesium forms an oxygenscavenging blanket the envelopes the component, reducing the oxygencontent within the part to negligible levels. Other metal powders thatmay be mixed with the ceramic support powder are zirconium, lithium,beryllium, calcium, cerium, lanthanum, neodymium, praseodymium,samarium, thorium, uranium, or misch metal and mixtures thereof, eitherin combination with magnesium, as a substitute for magnesium or incombination with aluminum.

Once the binder has been removed, the temperature is increased to atemperature, effective to promote the formation of aluminum nitride atlow oxygen partial pressures. An aluminum nitride skeleton forms on thesurface of the aluminum-base alloy powders. The aluminum nitrideskeleton is rigid and significantly increases the strength of thecomposite. However, because the skeleton is also rigid, an excessivelythick skeleton is not desirable due to the resultant decreasedductility. Since the rigid skeleton provides dimensional stability, itshould not be attacked by the liquid infiltrate.

A suitable thermal profile includes at least two isothermal holds. Thefirst isothermal hold occurs at a temperature of between 500° C. and570° C. for a time of from 2 to 20 hours. A preferred first isothermalhold is 540° C. for from six to 12 hours. The second isothermal hold isat a temperature above the melting temperature of the infiltrantinitiating infiltration 16. Nitridation 15 continues during the firsthold and an initial portion of the second isothermal hold and only stopswhen liquid infiltrant covers the exposed surfaces of the component.

A second suitable thermal profile is particularly effective to prevent arough surface on the component that has been compared to an orange peelappearance. The orange peel effect is most prevalent on parts havinglarge, flat, surfaces and is believed due to surface porosity andincomplete infiltration. The surface finish is improved when the processgas is changed from nitrogen to a noble gas after the first isothermalhold. Changing the gases stops nitridation 15 and eliminates the orangepeel effect. In accordance with this profile, the first isothermal holdis as above, at a temperature of between 500° C. and 570° C. for a timeof from 2 to 20 hours and preferably at 540° C. for from six to 12hours. Once nitridation 15 is complete, the process gas is changed to anoble gas, such as argon, helium or neon, and the system purged underthe noble gas for an effective amount of time, such as about one hour.Argon is preferred as the noble gas. The temperature is then increasedto the second isothermal hold which is at a temperature above themelting point of the infiltrant, held at temperature until infiltration16 is completed and then cooled to room temperature (nominally 22° C.).

The infiltrant is selected to melt at a temperature higher than thatrequired for skeleton formation. For the successful infiltration of afreestanding part (i.e. one not confined within a mould), it isnecessary to form a rigid and inert skeleton as this enables shaperetention during infiltration by the liquid. While it was originallypresumed that the infiltrant must melt at a temperature less than themelting point of the first aluminum-base alloy or dimensional stabilitywould be lost, it is now recognized that the AlN skeleton, having amelting point of about 3000° C., remains solid at any practicalinfiltration temperature and maintains the shape of the component. Themelting point of the infiltrant is therefore not important, as long asthe infiltrant melts at a temperature above that at which the AlN forms.By melt, it is meant the temperature at which a liquid phase initiallyforms, referred to by those skilled in the art as the solidustemperature.

In addition, the infiltrant must have sufficient fluidity and asufficiently low viscosity to flow through the interconnected pores ofthe composite. In addition, the contact angle between a bead of theinfiltrant and the skeleton must be sufficiently low to support goodwettability. A contact angle of greater than 90° is typically viewed asnon-wetting while a contact angle of less than 90° is viewed as wetting,with the closer to 0° the more effective the infiltration. Furtherconsiderations are the solubility of the aluminum alloy powder in theliquid infiltrant and the phase diagram of the combination of aluminumalloy powder and infiltrant. A large number of phases or a number oftransient phases is not desirable, since that could lead toinhomogeneity in the solidified composite.

The infiltrant may be aluminum or an aluminum based alloy furthercontaining one or more of the following: copper, magnesium, silicon,zinc, titanium, zirconium, iron, silver, lead, tin, bismuth, antimony,strontium, sodium and nickel. In addition to aluminum-base alloys,aluminum with up to about 33% by weight copper alloy is also acceptable.

As a non-exclusive list, the following alloys are useful as theinfiltrant. All compositions are specified in weight percent. Eachcomposition may contain other, unspecified elements in amount that doesnot materially affect the infiltration properties described above.Silicon  8%-18% Magnesium  3%-7% Aluminum balance. Nominal (Al—13.8%Si—4.7% Mg) melting temperature of 557° C. Copper 28%-38% Aluminumbalance. Nominal (Al—33% Cu) melting temperature of 548.2° C. Silicon 8%-12% Zinc  8%-12% Nickel  3%-8% Aluminum balance. Nominal (Al—10.5%Si—10% Zn—5.5% Ni) melting temperature of 549° C. Silicon  8%-18%Aluminum balance. Nominal (Al—12% Si) melting temperature of 577 ± 1° C.

Other suitable infiltrants are listed in Table 1. Each composition maycontain other, unspecified elements in amount that does not materiallyaffect the infiltration properties described above. TABLE 1 Infiltrant(Alloying Additions in Weight Percent, Solidus Liquidus Range BalanceAluminum) (° C.) (° C.) (° C.) Al—2.4 Zn 650 656 6 Al—2.9 Fe 654 660 6Al—2.2 Zn 639 653 14 Al—2.7 Mn 658 677 19 Al—2.3 Cu 604 654 50 Al—5 Mg580 635 55 Al—2 Cu—1 Mg 584 650 66 Al—1 Si—0.9 Mg—0.7 Mn 580 649 69Al—2.3 Si 577 647 70

Once at the infiltration temperature, generally at least about 10° C.above the melting temperature of the infiltrant, the part is held attemperature for a time effective for complete infiltration 16 of molteninfiltrant into the preform, on the order of about 1 to about 15 hours,and preferably from about 2 hours to about 10 hours. At which time thepart is cooled, typically at a rate of from about 1° C. per minute toabout 5° C. per minute, and nominally 2° C./minute, to solidify 18.

Following solidification, the strength of the part may be increased byheat treating the infiltrated part. One suitable heat treatment is toheat from about 500° C. to about 550° C. for from about 1 to about 24hours followed by a water quench. Additional strength is achievedthrough age hardening, either at room temperature (natural aging) or atelevated temperatures, typically at about 100° C. to about 200° C., fora time effective to promote full hardening.

Other post-solidification treatments may include hot isostatic pressingto close residual porosity and polishing or sand blasting to provide asmooth finish to the part.

The mechanism by which the Applicants successfully spontaneouslyinfiltrated an aluminum alloy with a different aluminum alloy isbelieved to be the following. This represents Applicants bestunderstanding of the process as of the filing of the patent application.With reference to FIG. 6, a particle of aluminum alloy powder 20 has ametallic core 22, such as, by weight, nominally Al-1% Mg-0.6% Si-0.25%Cu for aluminum alloy 6061. Surrounding the core 22 is a thin,chemically and thermally stable, alumina film 24. With reference to FIG.7, a blended mixture of powders 26 is formed. The mixture 26 includesaluminum or aluminum alloy particles 20 (the alumina film is present,but sufficiently thin not to be illustrated in FIG. 7), oxygen scavengerparticles 28, such as magnesium, and a polymer binder 30, such asnylon-12. As nominal quantities, there is about 2%, by weight of theoxygen scavenger and about 10% by volume of the binder with the balancealuminum alloy particles.

With reference to FIG. 8, the blend of powders of FIG. 7 is formed intoa desired near net shape, such as by rapid prototyping. Duringprototyping, the polymer binder 36 melts and fuses aluminum or aluminumalloy particles 20 and oxygen scavenger particles 28 to form a greencompact 34. The green compact is placed in a crucible 36 that readilywithstands temperatures up to about 800° C. As a non-limiting list,suitable materials for crucible 36 include porcelain and stainlesssteel.

A desired infiltrant 32, such as, by weight, nominally Al-13.8% Si-4.7%Mg, is placed in contact with the green compact 34. The green compact 34is then covered in a blanket 38 that is a mixture of oxygen reducingparticles and support particles. The oxygen reducing particles may be ametal or metal alloy and the support particles a ceramic. One exemplaryblanket is alumina (Al₂O₃) particles to which 1% by weight (2% byvolume) magnesium powder has been added. The Al₂O₃—Mg blanket serves twopurposes. First is mechanical support, required from the time the binderis burnt out and the time the aluminum nitride skeleton forms. Thesecond purpose is to provide an oxygen-free atmosphere at the surface ofthe part which is a key requirement for the formation of the aluminumnitride skeleton. The aluminum nitride skeleton has to form apercolating network across the entire part, including the outermostsurface. The aluminum nitride holds the part together duringinfiltration and also provides a wetting surface for the liquidaluminum. If the nitride does not form to the very edge of the part,then the edge particles are not held in place and the infiltrant doesnot penetrate to the part's surface. The outer layers of the greencompact are then easily removed after infiltration and dimensionalprecision is lost. It is therefore desirable that every particle iscovered by an aluminum nitride layer. The aluminum nitride will not formunless the oxygen partial pressure is exceedingly low. The requiredexceedingly low oxygen partial pressures are only achieved in thepresence of an oxygen getter. The outer layers of the green compactfunction as a getter for the inner layers. However, the outermost layersof the green compact cannot function as a self-getter and nitride doesnot form on these particles unless an external getter is provided.

Crucible 36 is inserted into a furnace 40 having a controlled atmospheresuch by way of gas inlet 42. The green compact 34 is then heated in aninert atmosphere, preferably nitrogen containing and more preferably,substantially only nitrogen to a temperature effective to melt thepolymer binder 30, without melting any of the metallic components(aluminum alloy powder 20, oxygen scavenger 28 and infiltrant 32). For anylon-12 binder, this temperature is in the range from about 150° C. toabout 300° C. As the blend of powders is further heated, such as throughthe temperature range of from about 300° C. to about 540° C. innitrogen, the polymer binder 30 begins to decompose. If the polymerbinder 30 is nylon-12, the binder decomposes to a carbonaceous residue,ε-Caprolactam (C₆H₁₁NO) and gaseous fixed nitrogen species such as HCN,N₂O and NH₃. Gaseous carbon species such as CO and CO₂ are also formed.

While the assembly may be moved to different ovens to achieve thedesired thermal exposures, it is preferred that the assembly remain in asingle atmosphere controlled oven programmed with temperatures and timeperiods sufficient to perform each process step in series.

With reference to FIG. 9, after the removal of the binder, the part isheld at a temperature between the lowest temperature at which thealuminum nitride compound forms and the temperature at whichinfiltration occurs in a nitrogen-containing atmosphere or thetemperature at which the first aluminum-base alloy melts, whichever isthe lower temperature. Preferably, the atmosphere is substantially onlynitrogen. By applying an isothermal hold in this temperature band, andproviding the oxygen content is sufficiently low, partial conversion ofthe aluminum to an aluminum nitride compound 36 occurs. Growth of thealuminum nitride compound results in the formation of a rigid skeleton.The hold time should be such as to allow sufficient but not excessiveformation of this skeleton. Typically, a hold time of about 2 hours atabout 540° C. is used. Once a skeleton has formed, the atmosphere ischanged from nitrogen to argon and the temperature is then increased toabove that at which the infiltrant 32 becomes molten to allowspontaneous or pressureless infiltration of the part. The part is heldat the infiltration temperature sufficiently long to ensure fullpenetration of the liquid, typically about 2 to about 12 hours.

While acceptable for robust parts having few sharp corners that inducethe concentration of strain gradients, the Al₂O₃—Mg blanket illustratedin FIG. 8 is not ideal for complex parts. When the Al₂O₃—Mg blanket isfirst added to crucible 36 to surround the green compact 34, the blanket38 is a free-flowing powder. After completion of the thermal cycle, thepowder is transformed into a rigid mass that prevents thermal expansionand shrinkage of the part during heating and cooling. The stressesresulting from this restraint on the part can be sufficient to causecracking.

A second problem with the Al₂O₃—Mg blanket is that it can form asignificant waste stream. By the end of the manufacturing process,excess magnesium has reacted with nitrogen gas to form Mg₃N₂. Themagnesium nitride then reacts with moisture in the atmosphere to formMgO plus NH₃ that is both volatile and toxic. The used blanket cannot bereused without being reformatted and it is difficult to recycle ordispose of the blanket. The used blanket therefore represents anundesirable by-product that may be eliminated or at least significantlyreduced in volume by the following alternative assembly.

As illustrated in FIG. 10, green compact 34 is placed on a thin layer ofAl₂O₃—Mg blanket rather than being completely enveloped in the material.As a result, only one side of the green compact is in substantialcontact with the blanket 44. The thickness of this Al₂O₃—Mg layer may befrom about one millimeter to about 10 millimeters and is nominally about5 millimeters.

Crucible 36 is covered with a lid 46 having at least one, and preferablya plurality, of perforations 48. The perforations are of sufficient sizeand number to be effective to remove binder vapor from within thecrucible. Apertures having a cumulative area that is from about 0.05% toabout 5%, by area, of the area of the blanket is preferred. For theexample of a lid covering a thin blanket layer 200 millimeters×200millimeters in size (40,000 millimeters square in area) it was foundthat three perforations each of about 3 millimeter in diameter (totalopen area of 42.4 millimeters square) was sufficient. The furnace 40 wasevacuated to a vacuum of approximately 0.1 torr prior to backfillingwith nitrogen and heating.

The magnesium component of the Al₂O₃ powder mix has a very high vaporpressure vaporizing at the nitridation temperature to function as anoxygen getter around the outside surfaces of the green compact 34.Unless the magnesium vapor is constrained in some way, it is dissipateddownstream with the nitrogen gas entering through gas inlet 42. Cruciblelid 46 is required to retain the magnesium vapor and to minimizeinteraction between incoming nitrogen gas and the green compact. Even ahigh purity nitrogen supply contains a few parts per million of oxygenas an impurity which is an oxygen level sufficiently high to prevent theformation of aluminum nitride.

The crucible 36 cannot be completely sealed and requires a loose fittingperforated lid 46. The number and size of the perforations is thateffective to allow removal of the binder and the ingress of nitrogen butinsufficient to allow the magnesium to dissipate. Use of the coveredperforated crucible causes the incoming gas to be largely isolated fromthe part and is mostly only gas replacing those gases that are consumedby the nitride reaction that filters through the holes and into thecrucible. The oxygen content of this gas is effectively reduced by themagnesium vapor. The vacuum is required to remove oxygen from thevicinity of the part prior to heating. Otherwise, oxygen contained inthe air depletes the magnesium powder and the continual gettering powerof the powder is lost. The evacuation step may be eliminated if freespace in crucible 36 is kept to a minimum.

FIG. 11 shows a surface of an infiltrated aluminum part subsequent toinfiltration. Aluminum or aluminum alloy particles 20 and aluminumscavenger particles 28 are joined together by a rigid aluminum nitrideskeleton (not shown). Disbursed between aluminum or aluminum alloyparticles 20 and aluminum scavenger particles 28 is infiltrant 32. Sincethe structure is somewhat porous, during infiltration some infiltrant 32may leak out of the part and form a pool 52, commonly referred to asbleed-out. Bleed-out is detrimental because it destroys the dimensionalaccuracy of the part. The bleed-out problem is particularly manifestwhen the surface of the part is not in contact with an aluminum blanketas in the embodiment illustrated in FIG. 10. While bleed-out may becontrolled by reducing the volume of infiltrant, this risks incompleteinfiltration resulting in either poor dimensional precision and/or poorproperties.

Bleed-out, even in the presence of excess infiltrant, is substantiallyeliminated by coating the exterior surfaces of the part subsequent toforming a green composite of a desired shape, but before infiltrationwith a material that is not wet by molten aluminum. This material shouldpreferably coat all exterior surfaces uniformly including hollowsections and undercuts. One particularly useful coating material isboron nitride, BN, such as ZYP BN aerosol provided by ZIP Coating, Inc.,Oak Ridge, Tenn. Aerosols containing other powders that are not wet bymolten aluminum, for example Al₂O₃, may also be used. A preferredthickness for the non-wettable powder it that which is effective toprevent bleed-out.

The above invention will become more apparent from the Examples thatfollow.

EXAMPLES Example 1

A green composite was made by selective laser sintering of a powdermixture containing 6061 powder, 2 wt % Mg and 10 vol % nylon binderusing each of the nylon binders previously recited as being commerciallyavailable from Atofina S.A. An infiltrant with a composition, by weight,of Al-13.8Si-4.9Mg was placed in contact with the preform. The amount ofthe infiltrant was sufficient to just fill the pore volume. The assemblywas then placed inside a crucible and covered with a support powderconsisting of alumina containing 1 vol % Mg powder. The crucible wasthen placed inside a nitrogen-atmosphere furnace and heated atapproximately 90° C. per hour to a temperature of 540° C. and held for aperiod of 2 hours to allow the skeleton to form. The furnace temperaturewas then increased at the same rate to 570° C. and held for 4 hours toallow spontaneous infiltration of the whole preform. The parts were thenfurnace cooled until the temperature was below 200° C. and then removedfrom the furnace and air-cooled. The parts were removed from the supportpowder and sand blasted. The density of each part was close to thetheoretical density for the aluminum-base alloy.

Similarly successful infiltrations were obtained by the processesrecited in Examples 2 through 7 that follow.

Example 2

An alloy was made and processed as per Example 1 but with an infiltrantcomposition of Al-33 wt % Cu.

Example 3

An alloy was made and processed as per Example 1, but with an infiltrantcomposition of Al-00.5Si-10 Zn-5.5Ni.

Example 4

An alloy was made and processed as per Example 1, but with an infiltrantcomposition of Al-12Si and an infiltration temperature of 590° C.

Example 5

An alloy was made and processed as per Example 1, but the initial powdermixture consisted of 6061 powder and 10 vol % nylon binder.

Example 6

An alloy was made and processed as per Example 1, but the initial powdermixture consisted of aluminum powder, 2 wt % Mg and 10 vol % nylonbinder.

Example 7

A green body consisting of a powder mixture containing 6061 powder, 2 wt% Mg and 10 vol % nylon binder using each of the nylon binderspreviously recited as being commercially available from Atofina S.A. wasmade by placing the powder mixture in a mould and heating this to atemperature above the melting point of the nylon. On cooling, theresin-body green body was extracted from the mould and processed as perExample 1.

Example 8

To confirm the hypothesis that the melting point of the infiltrant couldexceed the melting temperature of the base alloy, cast preforms wereproduced from a mixture of 2% by weight, magnesium powder, 1% by weighttin powder, balance aluminum alloy 6061 powder combined with 10%, byvolume, nylon. The mixture was nitrided for 12 hours at 540° C. innitrogen, purged for one hour in argon and then infiltrated withcommercially pure aluminum (99.7%) in argon at 700° C. for six hours.Dimensional accuracy was maintained and acceptable parts were produced.

Example 9

To confirm the hypothesis that the infiltrant may have a relative broadmelting range, cast preforms produced from a mixture of aluminum alloy6061 powder, 2% by weight magnesium powder, 1% by weight tin powder and10% by volume nylon, were nitrided for 12 hours at 540° C. in nitrogen,purged for 1 hour in argon and then infiltrated in argon at 700° C. forsix hours. The samples were then annealed in air for two hours at 410°C. and water quenched. When the infiltrant was Al-5% by weight, Mg(melting range of 55° C.), the ultimate tensile strength of the finishedpart was 155 MPa and the tensile ductility was 1.6%. When the infiltrantwas Al-14% by weight, Si-4% by weight, Mg (melting range of 0° C.), theultimate tensile strength of the finished part was 100 MPa and thetensile ductility was 0.5%. As a control, when the infiltrant was purealuminum, the ultimate tensile strength of the finished part was 103 MPaand the tensile ductility was 2%.

Example 10

A systematic study tested three variables: (1) covering the crucible;(2) evacuating the furnace; and (3) spraying with BN. Covering thecrucible consisted of placing a loose steel lid containing three small(each 3 millimeter in diameter) holes over the crucible. Evacuation ofthe furnace occurred during the first hour of heating and the vacuumlevel attained was approximately 10⁻¹ torr. Spraying the parts with BNconsisted of applying a uniform coating of ZYP BN aerosol. The detailsof other experimental conditions are given in Table 2. All parts wereprocessed by a thermal profile of heating at a rate of 90° C. per hourup to 540° C. in nitrogen, hold for 12 hours followed by a 30 minutepurge with argon and then heat at a rate of 90° C. per hour up to 700°C. in argon and hold for two hours. An aluminum alloy 6061 infiltrantwas kept at 80% of theoretical maximum loading to minimize bleed-out.TABLE 2 Sprayed Erosion during Bleeding out Condition Covered Evacuatedwith BN sand blasting of infiltrant Success A Yes Yes Yes No No Yes BYes Yes No No Yes No C Yes No Yes No No Yes D Yes No No No Yes No E NoYes Yes Yes No No F No Yes No Yes? Yes No G No No Yes Yes No No H No NoNo Yes Yes No

It should be noted that the present process is applicable to othermaterials and compositions, and one skilled in the art will understandthat the alloys, blend percentages, particle sizes, and temperaturesdescribed herein are presented as examples and not limitations of thepresent invention.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A method for the manufacture of a three-dimensional object having adesired shape, comprising the steps of: a). forming a mixture thatcontains a binder and at least one of aluminum or a first aluminum-basealloy into a green composite having said desired shape; b). removingsaid binder from said green composite by a process effective to maintainsaid desired shape as a porous preform structure; c). converting aportion of said aluminum to aluminum nitride while said porous preformstructure is supported by a mixture of oxygen reducing particles andsupport particles to thereby transform said green composite to a rigidskeleton of said desired shape with said porous structure; and d).infiltrating said porous preform structure at an effective temperaturewith an infiltrant that is selected from the group consisting of moltenaluminum and a molten second aluminum-base alloy to form saidthree-dimensional object with a near theoretical density.
 2. The methodof claim 1 wherein said mixture of oxygen reducing particles and supportparticles are selected to be a mixture of an oxygen scavenging metal ormetal alloy and a ceramic.
 3. The method of claim 2 wherein said ceramicis selected to be alumina and said oxygen scavenging metal or metalalloy is selected to be magnesium or a magnesium-base alloy in an amountof from 0.5% to 5% by weight (1.0% to 10% by volume).
 4. The method ofclaim 3 wherein said magnesium or magnesium-base alloy is present in anamount of from 0.7% to 1.2% by weight (1.4% to 2.4% by volume).
 5. Themethod of claim 4 wherein only one side of said porous preform structureis in substantial contact with said mixture of oxygen reducing particlesand ceramic support particles.
 6. The method of claim 5 including thestep of enclosing both said porous preform structure and said oxygenreducing particles and ceramic support particles in a container.
 7. Themethod of claim 6 including the step of covering said container with anon-hermetic lid.
 8. The method of claim 7 including providing saidcover with a plurality of apertures.
 9. The method of claim 8 includingthe step of sizing said apertures to have an area that is from about0.05% to 5% by area of said side of said porous preform structure. 10.The method of claim 1 wherein subsequent to step (a), but before step(d), surfaces of said porous preform substrate are coated with a powderthat is not wet by molten infiltrant.
 11. The method of claim 10 whereinsaid powder is selected from the group consisting of boron nitride andalumina.
 12. The method of claim 11 wherein said powder is boron nitrideapplied to a thickness effective to prevent bleed-out.
 13. The method ofclaim 7 wherein subsequent to step (c), but before step (d), surfaces ofsaid porous preform substrate are coated with a powder that is not wetby molten infiltrant.
 14. The method of claim 13 wherein said powder isselected from the group consisting of boron nitride and alumina.
 15. Themethod of claim 14 wherein said powder is boron nitride applied to athickness effective to prevent bleed-out.
 16. A method for themanufacture of a three-dimensional object having a desired shape,comprising the steps of: a). forming a mixture that contains a binderand at least one of aluminum or a first aluminum-base alloy into a greencomposite having said desired shape; b). removing said binder from saidgreen composite by a process effective to maintain said desired shape asa porous preform structure; c). converting a portion of said aluminum toaluminum nitride while said porous preform structure is supported byoxygen reducing particles and ceramic support particles on substantiallyonly one side of said porous preform structure to thereby transform saidgreen composite to a rigid skeleton of said desired shape with saidporous structure; and d). infiltrating said porous preform structure atan effective temperature with an infiltrant that is selected from thegroup consisting of molten aluminum and a molten second aluminum-basealloy to form said three-dimensional object with a near theoreticaldensity.
 17. The method of claim 16 wherein said oxygen reducingparticles and ceramic support particles are selected to be a mixture ofceramic and an oxygen scavenging metal or metal alloy.
 18. The method ofclaim 17 wherein said magnesium or magnesium-base alloy is present in anamount of from 0.7% to 1.2% by weight (1.4% to 2.4% by volume).
 19. Themethod of claim 18 including the step of enclosing both said porouspreform structure and said oxygen reducing particles and ceramic supportparticles in a container.
 20. The method of claim 19 wherein said firstaluminum base alloy is selected to be an alloy with copper, magnesium,silicon, zinc, titanium, chromium, zirconium, nickel, iron, manganese,or silver, and mixtures thereof.
 21. The method of claim 20 wherein saidbinder is selected to be a polymer that substantially decomposes togases at a temperature of between about 300° C. and about 500° C. in anitrogen-base atmosphere.
 22. The method of claim 21 wherein said firstaluminum base alloy is mixed with a nitride control agent.
 23. Themethod of claim 22 wherein said nitride control agent is present in anamount of from 0.1% to 2.0%, by weight.
 24. The method of claim 23including an addition of an oxygen scavenger to said mixture.
 25. Themethod of claim 24 wherein said oxygen scavenger is selected to bemagnesium.
 26. The method of claim 25 including the step of forming anitride skeleton within said porous preform by exposure to nitrogen atlow oxygen partial pressure.
 27. The method of claim 26 wherein saidinfiltrant is selected to be aluminum or an alloy of aluminum andcopper, magnesium, silicon, zinc, titanium, zirconium, iron, silver,lead, tin, bismuth, antimony, strontium, sodium, or nickel and mixturesthereof.
 28. The method of claim 27 wherein step (b) is at a temperatureof between about 300° C. and about 500° C., step (c) is at a temperatureof from about 500° C. to about 570° C. and step (d) is at a temperatureof in excess of about 540° C. and equal or greater than the melting orsolidus temperature of the first aluminum alloy.
 29. The method of claim28 wherein steps (b)-(d) are in a single oven programmed withtemperatures and times effective for each step and steps (b) and (c) areperformed in a nitrogen-base atmosphere and step (d) is performed in anoble gas atmosphere.
 30. The method of claim 29 wherein subsequent tostep (a), but before step (d), surfaces of said porous preform substrateare coated with a powder that is not wet by molten infiltrant.
 31. Themethod of claim 30 wherein said powder is selected from the groupconsisting of boron nitride and alumina.
 32. The method of claim 31wherein said powder is boron nitride applied to a thickness effective toprevent bleed-out.